The invention relates to a colour television display tube comprising an electron gun system of the "in-line" type in an evacuated envelope for generating three electron beams. The beam axes are co-planar and converge on a display screen provided on a wall of the envelope while the beams are deflected across the display screen into two orthogonal directions by means of a first and a second deflection field. The electron gun system is provided with field shapers for causing the rasters scanned on the display screen by the electron beams to coincide as much as possible. The field shapers comprise elements of a magnetically permeable material positioned around the two outer beams and placed adjacent the end of the electron gun system closest to the screen.
A colour television display tube of this type is known from U.S. Pat. No. 4,196,370. A frequent problem in colour television display tubes incorporating an electron gun system of the "in-line" type is what is commonly referred to as the line and field coma error. This error becomes manifest in that the rasters scanned by the three electron beams on the display screen are spatially different. This is due to the eccentric location of the outer electron beams relative to the fields for horizontal and vertical deflection, respectively. The Patent cited above sums up a large number of patents giving partial solutions. These solutions consist of the use of field shapers. These are magnetic field conducting and/or protective rings and plates mounted on the extremity of the gun system which locally strengthen or weaken the deflection field or the deflection fields along part of the electron beam paths.
In colour television display tubes various types of deflection units may be used for the deflection of the electron beams. These deflection units may form self-convergent combinations with tubes having an "in-line" electron gun system. One of the frequently used deflection unit types is what is commonly referred to as the hybrid deflection unit. It comprises a saddle line deflection coil and a toroidal field deflection coil. Due to the winding technique used for manufacturing the field deflection coil it is not possible to make the coil completely self-convergent. Usually such a winding distribution is chosen that a certain convergence error remains, which is referred to as field coma. This coma error becomes clearly noticeable in a larger raster (vertical) for the outer beams relative to the central beam. The vertical deflection of the central beam is smaller than that of the outer beams. As has been described, inter alia, in the U.S. Pat. No. 4,196,370 cited above, this may be corrected by providing elements of a material having a high magnetic permeability (for example, mu-metal) around the outer beams. The peripheral field is slightly shielded by these elements at the area of the outer electron beams so that these beams are slightly less deflected and the field coma error is reduced.
A problem which presents itself is that the correction of the field coma (Y-coma) is anisotropic. In other words, the correction in the corners is less than the correction at the end of the vertical axis. This is caused by the positive "lens" action of the line deflection coil (approximately, quadratic with the line deflection) for vertical beam displacements. (The field deflection coil has a corresponding lens action, but it does not contribute to the relevant anisotropic effect). The elimination of such an anisotropic Y-coma error by adapting the winding distribution of the coils is a cumbersome matter and often introduces an anisotropic X-coma.